Method for determining the speed of a terminal equipment and a receiver

ABSTRACT

A method and receiver for determining the speed of a terminal equipment in a radio system using a time division multiple access method in which a signal is transmitted in a succession of time slots each of a given duration and the signal is received and sampled, the received sampled signal having a signal envelope with a mean strength, in which the speed determination is made by measuring the man strength of the signal envelope of the received sampled signal within a given time window. The given time window has a duration that is at most half of the duration of a time slot, a parameter proportional to the mean strength of the signal envelope is formed during each time window, in each received time slot the parameter is measured by using at least two time windows, and a variation of the envelope is calculated by time slots on the basis of the parameter, to form a derived variation value, the derived variation value is averaged over several time slots to form an averaged variation value, and the averaged variation value is compared with a calculated reference value, which comparison indicates the speed of the equipment.

This application is the national phase of international applicationPCT/FI97/00102 filed Feb. 18, 1997 which designated the U.S.

FIELD OF THE INVENTION

The present invention relates to a method for determining the speed of aterminal equipment in a radio system using a time division multipleaccess method, which method comprises measuring the mean strength of theenvelope of a received sampled signal within a given time window.

BACKGROUND OF THE INVENTION

The present invention is suitable for use especially in cellular radiosystems and also in other digital radio systems where terminalequipments communicating with base stations move beyond cell boundaries.Information on the moving speed of the terminal equipment will make itessentially easier to manage resources of the radio system, such ashandover anticipation and power control optimization.

For example, in a system using small microcells and bigger, so-calledumbrella cells overlapping microcells, it is advantageous that fastmoving terminal equipments are connected to umbrella cells and slowlymoving or stationary terminal equipments to smaller microcells. In thisway it is possible to diminish significantly the number of requiredhandovers and thus signalling loading in the network.

One known solution for determining the speed of a mobile terminalequipment is to monitor Doppler shift of carrier frequency. This methodis, however, impractical as it requires a stable frequency referencesource which is expensive.

Another known method has been described in Doumi T., Gardiner J. G., Useof base station diversity for mobile speed estimation, ElectronicLetters, Vol. 30, No. 22, pp. 1835-1836. The method of the referencecited utilizes base station antenna diversity. The antenna providing thestrongest signal level is always selected for diversity. The frequencyshift of the antenna is proportional to Doppler shift on the basis ofwhich the speed of the equipment can be deduced.

U.S. Pat. No. 5,396,645 describes a method for measuring the speed of anequipment. In the method of the reference cited, variations of thestrength of a received signal are measured during a predetermined timeinterval and the speed of the equipment is deduced on the basis of thisvariation.

None of the known methods is, however, suitable in a TDMA radio systemwhere frequency hopping is used, that is, where the carrier frequencyused by the user of the equipments varies by time slots in accordancewith some given variance pattern.

CHARACTERISTICS OF THE INVENTION

The object of the present invention is to implement a determinationmethod of the speed of an equipment which is suitable to be used in TDMAsystems and in connection with frequency hopping as well as withoutfrequency hopping.

This will be attained by a method shown in the preamble which ischaracterized in that the given time window is at most half a time slotin length, that a parameter proportional to the mean strength of thesignal envelope is formed during each time window, that in each receivedtime slot the parameter is measured by using at least two time windows,and that the variation of the envelope is calculated by time slots onthe basis of the parameter, that the derived variation value is averagedover several time slots, and that the averaged variation value iscompared with the calculated reference value, which comparison indicatesthe speed of the equipment.

The invention also relates to a receiver in a radio system using a timedivision multiple access method, which receiver comprises means forsampling the received signal. The receiver of the invention ischaracterized in that the receiver comprises means for forming aparameter proportional to the mean strength of the signal envelopeduring a time window at most half a time slot in length, in eachreceived time slot by using at least two time windows, and means forcalculating the variation of the envelope by time slots on the basis ofthe parameter, and for averaging the derived variation value overseveral time slots, and means for comparing the averaged variation valuewith the calculated reference value, and for indicating the speed of theequipment.

The method of the invention has many advantages. The first preferredembodiment of the invention utilizing partial sample queues is suitablefor use in propagation conditions where a plurality of interferences ispresent in the radio channel. The second preferred embodiment of theinvention is suitable for use in a channel with less interference and itis correspondingly simpler to implement than the first embodiment.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the preferred embodiment of the invention will beexplained in more detail by means of the appended drawings, wherein

FIG. 1 illustrates an example of a cellular radio system where thesystem of the invention can be implemented,

FIG. 2 illustrates an example of the structure of the receiver of thefirst preferred embodiment of the invention,

FIG. 3 illustrates an example of the selection of the partial samplequeue,

FIG. 4 illustrates a diagram of calculating the partial sample queue,

FIG. 5 illustrates an example of the structure of the receiver of thesecond preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an example of a cellular radio system where thesystem of the invention can be implemented. The figure shows a set ofsmall, so-called microcells 102 to 106 and a big umbrella cell 100overlapping them. Each cell is typically served by a specific basestation equipment. Microcells are intended for slowly moving orstationary terminal equipments 110, for walkers, for example. Anumbrella cell serves fast moving terminal equipments 108 placed invehicles, for example. In order that the terminal equipments can bemaintained in correct types of cells, it is important to determine theirmoving speed. It should be noted that the method of the invention maynaturally be applied to other radio system types, such as the GSM systemwhere overlapping microcells and umbrella cells are not yet in use.

When a terminal equipment having a connection to a base station moves,the strength of the signal received from the terminal equipment by thebase station varies due to fast fading. It is previously known that thevariation of the signal envelope is proportional to the speed of theterminal equipment.

In a cellular radio environment, a radio channel may be described as anon-dispersive channel of a single propagation path or a multipathpropagating dispersive channel, depending on how the signal passingthrough the channel is delayed and spread in time domain. In the latterchannel, the multipath propagation of the signal and great delaydeviation lead to intersymbol interferences (ISI) of the receivedsymbols.

When a signal propagates through a dispersive multipath channel, mainlytwo interference types white Gaussian distributed noise (AWGN) andintersymbol interferences (ISI), can be detected in the envelope of thereceived signal. When a signal propagates in a non-dispersive singlepath channel, white noise is the main interference.

The first preferred embodiment of the invention will be examined first.This embodiment is suitable for use in an environment where a signalpropagates through a dispersive multipath channel. The preferred featureof the invention is that on the basis of it, both the effect of ISI andnoise can be removed from the signal envelope without any change to theeffect of fast fading on the envelope.

The solution of the invention will be now examined by means of the blockdiagram shown in FIG. 2. In the solution of the invention, speed isestimated on the basis of the variation of the envelope. In order thatthe effect of noise and ISS could be removed, partial sample queues andtheir means are employed in this embodiment of the invention. Because offrequency hopping, the variation of the envelope is examined within eachtime slot.

A received signal 200 is sampled in a prior art sampling means 202 andpartial sample queues are formed 204 in a manner to be explained below.The sampling speed may be one sample per each symbol, for example. Themean strength of the signal envelope is calculated by using a timewindow which is at most half a time slot in length. In each time slotthe mean strength of the envelope is calculated in a processor 208 byusing at least two time windows on the basis of which the mean variationof the envelope in the time slot is calculated. The mean envelopevariation value thus obtained is averaged further over several timeslots. By denoting the mean variation value of the envelope by D and thereceived signal by y(t,i) where i indicates the i^(th) time slot, thecalculation can be illustrated by the appended exemplary formula havingtwo time windows for a time slot: ##EQU1## where N=the number of timeslots,

M=the number of sample queues within one time slot,

j=the j^(th) sample queue,

y_(queue1) (t,j,i)=the mean comprised by the sample queue in the firsttime window, and

y_(queue2) (t,j,i)=the mean comprised by the sample queue in the secondtime window.

The averaged variation value D is compared in a processor 210 in advancewith theoretical reference values stored in some memory element 212, asa result of which comparison a speed 214 of the terminal equipment willbe found out. In a practical implementation the processors 208 and 210may be the same processor or a different processor, or a correspondingcalculation may be realized by dedicated logic, as is evident to thoseskilled in the art.

We shall next examine the forming of partial sample queues in a signalwhich is first phase modulated. As in the GSM system, an interferencecaused by ISI is a result of symbols with different phases addingtogether. As ISI affects only a finite number of adjacent symbols andphase modulation has a finite number of phases (such as four phases inMSK), it may be seen that the sample points of some envelope have gonethrough an equally slow fading. The samples with an equally fast fadingare selected as points of the partial sample queue. In this case thereis no interference caused by ISI within the partial sample queue.

The forming of a partial sample queue is illustrated in FIG. 3. Thefigure shows some of the samples in one time slot. The index of thesamples is on horizontal axis 300, the strength of the envelope onvertical axis 302 from which the effect of noise is removed. In theexample shown in the figure, a group of sample points 304 to 310 hasgone through a similar fast fading, for which reason a partial samplequeue 312 can be formed of them. Similarly, part samples queues 312 and316 can be formed of other points. The sample numbers of differentpartial sample queues may be unequal.

Because of noise, partial sample queues cannot be selected directly froma received sampled signal but the signal has to be regenerated beforethe partial sample queue is selected. The regeneration of a signal isillustrated in the diagram of FIG. 4. The diagram shows more preciselythe contents of block 204 in FIG. 2. The sampled signal is carried to adetector 400 which makes a hard decision for detecting the symbol. Harddecisions 202 are conveyed to a burst modifier 204 from which the burstformed of the symbols is further conducted to a modulator 406. Thesignal thus obtained is carried to a calculation processor 412 in whichan estimated channel function 410 is received from the detector as asecond input. The processor carries out a convolution between themodulated signal and the channel function, from which a regeneratedsignal 414 is derived. This signal is further conveyed to a calculationprocessor 416 that searches the partial sample queues.

The found partial sample queues are mapped onto the envelope of thereceived signal. The following example will be examined. It is assumedthat the received signal is y(t,i) and the regenerated signal is s(t,i),both containing 148 samples [y₁, y₂, . . . y₁₄₈ ] and [s₁, s₂, . . .s₁₄₈ ] in one time slot. It is assumed that a partial sample queue [s₅=s₂₃ =s₈₉ =s₁₂₁ ] has been found from the regenerated sample queue. Thismeans that s₅ =s₂₃ =s₈₉ =s₁₂₁. In accordance with the formula of D shownabove then

    y.sub.queue1 (t,j,i)=(y.sub.5 +y.sub.23)/2 and

    y.sub.queue2 (t,j,i)=(y.sub.89 +y.sub.121)/2.

We shall now examine the second preferred embodiment of the inventionsuitable for use in an environment where a signal propagates through anon-dispersive single path channel. The preferred feature of theinvention is that its implementation is simpler than that of the abovesolution utilizing partial sample queues.

Let us examine the solution of the invention by means of the diagramshown in FIG. 5. In the solution of the invention, the speed isestimated on the basis of variation of the envelope. The effect of ISIis not taken into account in this solution model as it is not present ona non-dispersive channel. As above, because of frequency hopping,variation of the envelope is examined within each time slot.

Samples 502 are taken from the received signal 200. The sampling speedmay be, for example, one sample for each symbol. The mean strength ofthe signal envelope is calculated by using a time window which is atmost half a time slot in length. The mean strength of the envelope iscalculated in each received time slot by using at least two timewindows, on the basis of which the mean variation of the envelope in thetime slot is calculated in a processor 500. The mean envelope variationvalue thus obtained is averaged further over several time slots. Bydenoting the mean variation value of the envelope by D and the receivedsignal by y(t,i) where i indicates the i^(th) time slot, the calculationcan be illustrated by the appended exemplary formula having two timewindows for a time slot: ##EQU2## where N=the number of time slots,

y_(queue1) (t,j,i)=the mean in the first time window, and

y_(queue2) (t,j,i)=the mean in the second time window.

The averaged variation value D is compared in the processor 210 inadvance with some theoretical reference values stored in the memoryelement 212, as a result of which the speed 214 of the terminalequipment is found out. In a practical implementation the processors 500and 219 may be the same processor or a different processor, or acorresponding calculation may be implemented by dedicated logic, as isevident to those skilled in the art.

As distinct from the embodiment presented first, in this implementationthe strength of the envelope is calculated by means of all the samplesplaced in the time window, whereas partial sample queues were formed inthe first embodiment.

The structures shown in both FIG. 2 and FIG. 5 are examples of receiverstructures of the invention only in parts essential for the invention.The receivers of the invention obviously comprise other components thanthe ones shown in the figure, such as an antenna, radio frequency partsand filters, as is evident to those skilled in the art.

Although the invention has been above explained with reference to theexample of the accompanying drawings, it is evident that the inventionis not restricted thereto but it may be varied in many ways within theinventive idea disclosed in the appended claims.

What is claimed is:
 1. A method for determining the speed of a terminalequipment in a radio system using a time division multiple access methodin which a signal is transmitted in a succession of time slots each of agiven duration and the signal is received and sampled, the receivedsampled signal having a signal envelope with a mean strength, whichmethod comprises measuring the mean strength of the signal envelope ofthe received sampled signal within a given time window, characterized inthatthe given time window has a duration that is at most half of theduration of a time slot, a parameter proportional to the mean strengthof the signal envelope is formed during each time window, in eachreceived time slot the parameter is measured by using at least two timewindows, and a variation of the envelope is calculated by time slots onthe basis of the parameter, to form a derived variation value, thederived variation value is averaged over several time slots to form anaveraged variation value, and the averaged variation value is comparedwith a calculated reference value, which comparison indicates the speedof the equipment.
 2. A method according to claim 1, characterized inthat the parameter proportional to the mean strength of the signalenvelope is calculated by forming one or more partial sample queues ofthe samples of the received signal, the mean strength being calculatedfor each partial queue.
 3. A method according to claim 2, characterizedin that each partial sample queue is formed of the samples of theenvelope whose fast fading is equal.
 4. A method according to claim 3,characterized in that the partial sample queue is formed of aregenerated signal in which decision, modulation and multiplication ofchannel impulse response have been carried out.
 5. A method according toclaim 1, characterized in that the parameter proportional to the meanstrength of the signal envelope is calculated by averaging the measuredstrength of all the samples in the time slot.
 6. A receiver fordetermining the speed of a terminal equipment in a radio system using atime division mutiple access method in which a signal is transmitted ina succession of time slots each of a given duration, which receivercomprises means for receiving the transmitted signal and sampling thereceived signal such that the received sampled signal has a signalenvelope with a mean strength, characterized in that the receivercomprisesmeans for forming a parameter proportional to the mean strengthof the signal envelope during at least two given two windows each havinga duration at most equal to half the duration of a time slot, in eachtime slot of the received signal, means for calculating the variation ofthe envelope by time slots on the basis of the parameter to form aderived variation value, and for averaging the derived variation valueover several time slots, and means for comparing the averaged variationvalue with a calculated reference value, and for indicating the speed ofthe equipment.
 7. A receiver according to claim 6, characterized in thatthe receiver comprises means for forming one or more partial samplequeues of the received signal, and means for calculating the meanstrength for each partial queue.
 8. A receiver according to claim 7,characterized in that the receiver comprises means for forming eachpartial queue of the samples of the received signals whose fast fadingis equal.
 9. A receiver according to claim 8, characterized in that themeans for forming one or more partial sample queues comprise means fordetecting the received signal, means for assembling bursts of detectedsymbols, means for modulating the assembled bursts, means formultiplying the modulated assembled bursts by channel impulse responseand means for finding the partial sample queues from the multipliedsignal.
 10. A receiver according to claim 6, characterized in that thereceiver comprises means for measuring the strength of all of signalsamples in each time slot, and in that that the means for forming aparameter proportional to the mean strength of the signal envelope formsthe parameter by averaging the measured strength of all the samples ineach time slot.